This invention relates generally to flowmeters of the vortex type, and more particularly to a vortex meter associated with an electronic data processing system adapted to provide highly accurate mass flow or volumetric flow measurements over a broad fluid viscosity range.
In many industrial processes, one must be able to measure the volumetric flow of fluids being treated or supplied in order to carry out various control functions. It is also necessary, in some instances, to determine the mass flow of the fluids. Existing types of vortex flowmeters are capable of effecting volumetric flow or mass flow measurement.
It is well known that under certain circumstances the presence of an obstacle in a flow conduit will give rise to periodic vortices. For small Reynolds numbers, the downstream wake is laminar in nature, but at increasing Reynolds numbers, regular vortex patterns are formed. These patterns are known as Karman vortex streets. The frequency at which vortices are shed in a Karman vortex street is a function of flow rate. In order to convert a volumetric reading to a reading of mass flow, one must multiply the volume measurement by the density of the fluid being measured.
An improved form of vortex-type flowmeter is disclosed in Burgess U.S. Pat. No. 3,589,185 wherein the signal derived from the fluid oscillation is relatively strong and stable to afford a favorable signal-to-noise ratio insuring accurate flow-rate information over a broad range. In this meter, the obstacle assembly mounted in the flow conduit is constituted by a block positioned across the conduit with its longitudinal axis at right angles to the direction of fluid flow, a strip being similarly mounted behind the block and being spaced therefrom to define a gap which serves to trap Karman vortices and to strengthen and stabilize the vortex street. This vortex street is sensed to produce a signal whose frequency is proportional to flow rate.
In Herzl U.S. Pat. No. 3,867,839, the disclosure of which is incorporated herein by reference, in lieu of a thermistor sensor of the type disclosed in the Burgess patent, use is made of a strain gauge sensor. In the Herzl arrangement, the obstacle assembly is formed by a front section mounted across the flow tube and a rear section spaced from the front section by means of a flexible web to define a gap serving to trap the Karman vortices. Because the rear section, which is cantilevered from the front section, is deflectable, it is excited into vibration by the vortices at a rate whose frequency is proportional to fluid flow. This vibratory motion is sensed by the strain gauge sensor, which is elastically suspended in a cavity within the cantilever structure to produce a signal indicative of flow rate.
In a vortex type flowmeter, the meter factor (i.e., the number of cycles generated per gallon of fluid passing through the flow tube) is fairly constant with changes in Reynolds number, but only within the normal operating range of the meter. When applied to the flow of fluid in a pipe, Reynolds number equals the product of density, velocity and pipe diameter divided by the coefficient of the fluid viscosity.
The calibration curve for a vortex-type flowmeter is produced by plotting the Reynolds number of the flow tube against the meter factor. In the typical curve for a vortex meter, the meter factor (cycle/gal.) is relatively unchanged within the normal viscosity range. For example, the viscosity of water at 60.degree. C. is 9.89, at 80.degree. C. it is 7.42 and at 100.degree. C. it is 5.92. Assuming that these viscosity values lie within the normal range of the instrument, the meter factor will be substantially independent of changes in water viscosity resulting from variations in temperature between 60.degree. C. and 100.degree. C., and the reading of the meter will reflect flow rate with reasonable accuracy.
But if the viscosity of the water being measured lies outside of the normal range, then the meter factor will rise sharply as the viscosity increases. Thus assuming the normal range ends at a viscosity value of 21.0, the meter will be non-linear and inaccurate in a range including a water temperature of 20.degree. C. producing a viscosity value of 21.1, a water temperature of 10.degree. C. resulting in a viscosity value of 27.2 and a temperature of 0.degree. C. producing a viscosity of 37.5. In this abnormal range, the meter factor will be both a function of flow rate and fluid viscosity.
Thus to obtain accurate flow measurement in a broad viscosity range encompassing both that portion of the calibration curve where the meter factor is fairly constant with changes in Reynolds number and the remaining portion where a change in Reynolds number produces a marked change in meter factor, a correction for viscosity is essential. Similarly, accurate flow measurement is impossible without a density correction for changes in temperature which gives rise to changes in fluid density. The need for correction is even more pronounced in vortex meters for measuring hydrocarbon fluids, for oils exhibit large changes in viscosity and density with changes in temperature.